Device for controlling the on &amp; off time of the metal oxide semiconductor field effect transistor (MOSFET), a device spark coating the surfaces of metal workpiece incorporating the said control device and a method of coating metal surfaces using the said device

ABSTRACT

The present invention is a device for coating surfaces of metallic work pieces with an electrically conductive material by employing short duration high current packets of pulses in which the work piece forms the cathode and the consumable coating material forms the anode, which are connected to a generator for generating pulses by charging and discharging a bank of capacitors using a MOSFET. The invention is also a device for controlling the on and off time of a metal oxide semiconductor field effect transistor (MOSFET)

FIELD OF INVENTION

The invention relates to a device for controlling the on & off time ofthe metal oxide semi conductor field effect transistor (MOSFET), adevice for spark coating the surfaces of metal work piece incorporatingthe said control device and a method of coating metal surfaces using thesaid device. The present invention particularly relates to a deviceproducing thicker metallic, carbide and cermet composite coatings onelectrically conductive metallic substrates and method of coating usingthe said device. The present invention more particularly relates to adevice for producing thicker and uniform coatings on metallic substratesby electrospark coating, which is also known as electro spark alloyingand a method for coating said substrates.

The coatings obtained by using the device of the present invention havelower surface roughness, improved tribological properties and excellentwear resistance. The present invention also relates to a device forcarrying out the above-mentioned process.

BACKGROUND OF THE INVENTION

Metals, especially different steels and their alloys, and morespecifically high speed steels (HSS) are commercially used in cuttingtool industries as structural members or tools as also in engineeringindustries in applications widely ranging from kitchen knives to turbinecomponents. Use of these materials is dictated by the cost and strengthrequirement. Though available in abundance at affordable prices, thesematerials have certain limitations such as less wear resistance, itsdegree depending on the percentage of carbon and other alloyingelements.

Heat treatment of steels is a well-known process of hardening, which isin use for the last 100 years or more. The hardened layer provideshigh-level protection against wear, tear and corrosion. Various hardfacing techniques such as welding, laser hardening, plasma spraying,high velocity oxy fuel spraying, detonation spraying have been welldeveloped and widely used by engineering industries to produce hard wearresistant coatings. Thin coating techniques such as physical vapordeposition (PVD) and chemical vapor deposition (CVD) are also developedfor improving service life of cutting tools and other engineeringcomponents.

All these coatings are essentially employed on components, made ofdifferent metals and alloys, to combat various forms of wear, tear andcorrosion to enhance their service life. However, most of thesetechniques demand a high degree of pre-coating and post-coatingoperations that are often not cost effective. Moreover, these techniquesproduce large quantities of heat thereby forming heat-affected zones,which can lead to component warping, dimensional change and rejection ofthe components.

Size, shape and complexity of geometry of engineering components dorestrict the applicability of thermal spray techniques. Moreover, thesetechniques require high quality and costly powders such as TungstenCarbide-Cobalt, Chromium Carbide-Nickel Chrome, prepared by speciallydeveloped manufacturing routes such as atomization, fusing, sintering &crushing, chemical reduction and blending. Deposition efficiency ofthese powders is mostly less than 60%.

Hard facing is a term that refers to the deposition of filler metal onthe surface of a work piece to improve its wear properties againstabrasion, impact, erosion, galling & cavitation. It can also play animportant role in enhancing the surface properties of a material to suitservice conditions that impose an upper limit on the dimensions of apart. Cladding, surfacing, build up and buttering are some of the othercategories that represent hard facing techniques.

Cladding is a process in which a relatively thick layer of filler metalis produced on a carbon or low-alloyed steel substrate to improve itscorrosion resistance against such unfriendly atmospheres. Buildup refersto the addition of a weld metal to a base metal surface or to the edgeof the joint or to a previously deposited weld metal for restoring thecomponent dimensions to the required values. Buttering refers to theaddition of one or more layers of a weld metal to the face of the jointor surface to be welded. Unlike build up, buttering is carried out formetallurgical reasons and not for dimensional control. Buttering is usedespecially for joining dissimilar metals and when stress relieving ofthe complete weld is not desirable.

Surfacing is generally used to improve, repair and rework a part so thatit will have properties better than those of the actual part itself Inmost of such cases, chemical compositions and mechanical properties haveto be carefully considered, as they may be different for the surfacingmaterial and the base material. Furthermore, dilution, which is definedas the percentage ratio of work piece melted to the total sum of thefiller material and work piece material can also be an importantconsideration.

As most of the surface modification processes discussed above employsome means of heating, they lead to severe thermal stresses, which maywarp or damage the work piece and thereby damage the surface produced.In addition to the above considerations, all these surface modificationprocesses must be economical and be capable of being carried out inhouse.

In order to avoid the major problems associated with the above processeswithout sacrificing the advantages associated with welding, sparkcoating has been developed. This is also known by other names such as“Spark Alloying”, “Spark Hardening”, “Spark Toughening”, “PulsedElectrode Surfacing” and “Electrospark Deposition”.

The Electro Spark Coating (ESC) method is well known and the essence ofthis method is as follows: Current pulses are generated between aprocessing electrode and the work piece, which are periodically broughtin contact with each other. As they approach each other, at a particularmoment, the breakdown of the inter electrode gap takes place and anelectric discharge is produced, through which the energy stored incapacitors is released. This spark discharge results in erosion andresults in the transfer of the material of the processing electrode ontothe work piece i.e. it produces a surface coating and forms a modifiedlayer on the surface of the work piece.

By sweeping the electrode over a selected zone of the work piece, ametallurgical modification such as surface alloying or hardening of theselected area in the work piece can be obtained. In this technique, boththe consumable electrode i.e. coating material and the job i.e. workpiece, should be electrically conductive as both of them are connectedto the positive and negative terminals of a DC supply. The material tobe deposited forms an electrode, which is mounted on an electromagneticvibrator. The vibrations that are generated by the electromagneticvibrator produce the periodic make and break of the contact between theconsumable electrode and the work piece, which therefore initiate anddefine the rate and frequency at which the pulse discharges occur. Thesevibrations that are generated at the electromagnetic vibrator alsoprevent welding of the consumable electrode that is mounted on it to thework piece.

Auxiliary movements such as rotation of the electrode about its axis anda combination of rotation and linear vibrations have also been explored.

The phenomena of transferring a consumable electrode material on to asubstrate material by means of short duration electrical discharges hasbeen carried out in a variety of ways over several decades. Thefollowing references are representative of such coating processes.

BRIEF DESCRIPTION OF THE DRAWINGS OF PRIOR ART

FIG. 1 of the drawing accompanying this specification represents theschematic view of prior circuitry of Electrospark coating mentioned as aprior art in U.S. Pat. No. 4,405,851

FIG. 2 represents the schematic view of apparatus and circuitry fortransfer of metallic materials by electric discharge using directcurrent source protected in the U.S. Pat. No. 4,405,851);

FIG. 3 represents the schematic view of the apparatus and circuitry fortransfer of metallic materials by electric discharge using alternatingcurrent source protected by the U.S. Pat. No. 4,405,851

The prior art referred to in the U.S. Pat. No. 4,405,851, year 1983,discloses a common method for depositing hard anode material on to thecathode work piece through electro sparking. The device of the prior artworks on a resistance-capacitance (RC) relaxation circuit shown in FIG.1, employing a vibratory electrode holder (1) to make and break thecontact between an anode (4) (consumable electrode) and a cathode (2)(work piece) at regular intervals. When direct current DC (6) issupplied to the circuit resulting sparks between anode cathode pairmelts and spray-deposits, part of the electrode (4) on to the work piece(2). A capacitor (3) is used in parallel with a direct current source(6) for producing a high-energy short duration spark. A ballast (5) isconnected between the positive lead of the circuit and the electrode(4). The negative lead of the circuit is connected to the work piece(2).

It was assumed that the molten metal produced from the high temperaturespark is transferred from the anode (4) to the cathode work piece (2) ofthe circuit possibly by an expanding gas bubble. Although similar towelding, this material transfer involves a complex mechanism. during theprocess of material transfer, the material reacts with the atmospherewhere the coating is being applied e.g. nitrogen and oxygen, from theair in the atmosphere. The desired high-energy short duration spark isproduced by connecting a capacitor (3) in parallel with the directcurrent source (6). The value of the resistor (5) must be large enoughto prevent arcing after breakdown (capacitor discharge) has beeninitiated across the gap. A large resistance prevents arcing but reducesthe maximum energy that can be released from the capacitor. This is theinherent difficulty with this process of depositing thicker coatings.

Further disclosure in a modified circuit of their invention in U.S. Pat.No. 4,405,851, shows that their circuit can work with either a singlephase or a three-phase alternating current supply. Instead of a ballastor load resistor (5) as shown in FIG. 1, a high-speed thyristor switch(10) is used to control the capacitor discharge. High frequencyswitching can be used to open and close the capacitor-workpiece-electrode circuit. The circuit shown in FIG. 2 includes a clamp(13) for holding and oscillating the anode (12) about a central axis onthe work piece (14). The switching device (9) is also provided forrepeatedly switching the circuit on & off at a predetermined rate. Anindependent trigger pulse generator is supplied & connected to thethyristor for producing selectively spaced and controlled pulses inseries with the positive lead. In this circuit, the capacitance value isreported to be in the range of 2 to 800 μF. Two Diodes (7) & (8) areused to bypass and short out the reverse current through theelectrode-work piece contact area & connecting leads and protect thethyristor against reversals.

The device shown in FIG. 3 uses a current transformer (15) with primarywinding connected to a three-phase alternating current supply. Thesecondary winding is connected to a variable transformer (16) to controlthe voltage at electrode [anode] (21). The Secondary winding isconnected to the rectifier (R) that provides the current for chargingthe single storage capacitor. The outputs of all the three rectifiersare connected in parallel to the single capacitor (3).

The voltage requirement of the circuits shown in FIG. 2 and FIG. 3 rangefrom 60V to 120V and these devices produce coatings with a surfaceroughness of 40 μm R_(a) or more. The working voltage range, mentionedabove, is hazardous for the operator and precautions must be taken toavoid accidents. Likewise, the surface roughness values areun-acceptable for most applications in an as-coated condition. Althoughfinishing operations such as polishing and buffing can reduce theroughness, the consequent reduction in the coating thickness rendersthem unsuitable for any application.

U.S. Pat. No. 4,346,281, year 1982, discloses a method and apparatus forthe surface treatment of metallic work pieces using a multipleelectrode-rotating tool. The difference between the device of thispatent as compared to the device mentioned above is the use of multipleanodes.

Slapping contact of each of the electrode with work piece results inspark discharge and hence coating when both of them are connected to thepositive and negative poles of the direct current power supply. Theelectrode should be less than 2 mm in diameter and the length up to 8mm. Chance of electrode fracture is very high as the speed range is veryhigh from 50 to 20,000 rpm. Spark duration was 60 microseconds andcoatings were deposited with the help of rotating tool electrode holder.Working voltage was not mentioned while deposition rate mentioned was 22mg/min.

U.S. Pat. No. 4,866,237, year 1989, discloses another apparatus and amethod where longitudinal vibration, in addition to travel of theelectrode along a path, which is not perpendicular to but has a properslant angle with the work piece, was provided in order to get a motionof slow approach and subsequent moving away of the electrode (anode)from the work piece. Capacitance values were not mentioned whilst theworking voltage was mentioned as 50 V, pulse duration 200 microsecondsand coating thickness was limited to less than 50 micrometers. Tubularconsumable electrode was employed for obtaining maximum deposition.

U.S. Pat No. 5,448,035, year 1995, discloses yet another method andapparatus for pulse fusion surfacing where the electrode holder rotatesand oscillates the electrode simultaneously to increase the quantity ofthe deposit. The spark rate was varied according to various processparameters. Thyristor fired trigger/discharge resistance capacitancecircuit and a stepper motor were controlled by a microprocessor. Thethyristor-controlled circuit works at a voltage range of 40-70 V. Singlepulse train produces coatings whose thickness was limited to 25micrometers with tungsten carbide cobalt electrode. Oscillatingfrequency, rotational direction and speed of electrode can be controlledto achieve coatings with predetermined morphology and thickness on workpiece.

U.S. Pat. No. 5,980,681, year 1999, describes a process for treatment ofmetal work piece surface by electrical discharges. The process wasdesigned in such a way that microscopic melting of the work piecesurface was avoided while ensuring extinction of the individual aredischarges. This process was employed for roughening a metal work piecesurface. Pulses of 50 A current and 20 micro second duration wereemployed.

U.S. Pat. No. 6,020,568, year 2000, describes an electromechanicalprocess and apparatus for metal deposition using a direct currentsource. This apparatus basically includes an ultrasonic generator, awork piece holder, mounting attachments for holding the material to becoated on ultrasonic horn. The main difference being the use ofultrasonic generator instead of vibrating electrode holder. The coatingthickness was limited by 5 micrometers/pass. The frequency of ultrasoundmay be in the range of 10 kHz to 40 kHz.

U.S. Pat. No. 6,417,477 B1, year 2002, discloses a method and apparatusfor Electrospark alloying. Externally cooled special collet wasincorporated to hold the consumable electrode for improving thedeposition rate. While working voltage was 120 V, total capacitancevalue mentioned was 40 microfarads. A rotating tool electrode holder wasemployed for depositing the coatings in controlled atmosphere such asargon and vacuum. The deposition rate was mentioned to be 2.5 mg/min inair while the same is reduced by ⅓ in controlled atmosphere.

U.S. Pat. No. 6,835,908 B2, year 2004, describes still another methodand apparatus for controlling Electrospark deposition using electricalvariable wave forms from the Electrospark deposition process as a feedback parameters for optimizing the contact force between consumableanode and work piece cathode. The plurality of amplitudes of series ofelectrical energy pulses delivered to consumable electrode and workpiece pair were measured and correlated to the contact force betweenelectrode tip and work piece. With such set up it was claimed the flawedareas such as pits and groves can also be effectively coated. However,deposition rate and other electrical parameters were not revealed.

To sum up, the above prior art methods and apparatus employ higherworking voltages ranging from 60 to 120 V or even more, a single pulsetrain (one pulse with a fixed width), lower pulse durations (less than100 μsecond), higher current and lower capacitance values and thyristorsor thyratrons as the pulses switching devices. All these factors lead tolower deposition rates and higher coating roughness. It is also clearfrom the above prior art information that the coating thickness islimited to at the most 50 microns when carbide materials are depositedon steel substrates. Moreover, the improvement in performance parametersdue to these coatings is reported to be less than or equal to 500 timesthat of the bare sample when tested in pin on disk test rig. Theresulting coatings are also not found to be uniform.

In summary the conventional spark deposition, whether they arevibratory, rotatory or multi electrode holder, are undesirably limitedto achieve satisfactory results with regards to the thickness of thecoating/deposit, rate of deposition and the treated surface roughnessand uniformity of the deposited layer.

In spite of the wide variety of earlier processes, there is still scopefor improving the thickness and reducing the surface roughness of theElectrospark coatings that can be deposited on any electricallyconducting work pieces for example steels, their alloys and super alloysso as to increase the spectrum of application using this technique.

The main drawback of the earlier devices was that the roughness of theprocessed surface is a sizable fraction, ¼ to ½ or more, of the averagethickness of the total deposited coated layer, which results in a poorerquality of the obtained surface. Moreover, the earlier devices do notpermit a uniform increase in the thickness of the deposited layer asadditional pulses can only result in the generation of sparks from thepeaks of the previously deposited top layer, which can worsen thesurface roughness even more.

The present invention is directed towards improving the thickness andreducing the surface roughness of the Electrospark coatings that can bedeposited on any electrically conducting work pieces as well eliminatingthe difficulties of the prior art devices.

Considering these objectives and to meet the present day need forcoatings with improved coating thickness, and better tribological,electrical and wear resistance properties, research work for developingimproved Electrospark coating methods and devices has gained importance.

OBJECTIVES OF THE INVENTION

Therefore, the main objective of the present invention is to provide animproved device for spark coating the surfaces of metal work piece byelectrical discharges overcoming the drawbacks of the hitherto knowndevices

Another objective of the present invention is to provide an improveddevice for spark coating on the surface of electrically conductivemetals and their alloys, in particular steel and its alloy bodies forobtaining dense, hard, uniform and thick coatings with increased rate ofdeposition and low surface roughness consistent in operation for desiredresults and allows selection of operation parameters depending up on theelectrode and work piece material combinations.

Still another objective of the present invention is to provide animproved device for controlling the on & off time of Metal Oxide Semiconductor Field Effect Transistor (MOSFET)

Yet another objective of the present invention is to provide an improvedmethod for spark coating the surfaces of metal work piece by electricaldischarges employing the device of the present invention

Still another objective of the present invention is to provide animproved method for spark coating on the surface of electricallyconductive metals and their alloys, in particular steel and its alloybodies at very low working voltage for obtaining dense, hard, uniformand thick coatings using the device of the present invention.

Another objective of the present invention is to an improved method forspark coating on the surface of electrically conductive metals and theiralloys, in particular steel and its alloy bodies, to protect themagainst wear, corrosion, and oxidation using electrical discharges byemploying the device of the present invention.

Another objective of the present invention is to provide an improvedmethod for spark coating on the surface of electrically conductivemetals and their alloys, in particular steel and its alloy bodies, toprotect them against wear, corrosion, and oxidation using electricaldischarges, which is simple and economical using the device of thepresent invention.

The above objectives of the present invention have been achieved,according to the present invention, based on our observations, due toextensive R&D work that it would be advantageous to control capacitordischarge by a suitable high-speed Metal Oxide Semi conductor FieldEffect Transistor (MOSFET) switch, which opens and closes the work pieceelectrode circuit (total circuit). By controlling the on & off periodsof the MOSFET, the discharge rate of the capacitors can be varied over awide range. Moreover a new pulse concept known as “packets” isintroduced to significantly reduce surface roughness and improve thedeposition rates.

The device of the present invention is described with reference to FIGS.4 & 5 of the drawings accompanying this specification.

FIG. 4 shows the device (circuit) of the present invention forgeneration and controlling pulses in packets to drive and control the on& off time of Metal Oxide Semi conductor Field Effect Transistor(MOSFET) of the present invention.

FIG. 5 shows the device of the present invention incorporating thedevice for the generation and controlling pulses in packets to drive andcontrol the on & off time of Metal Oxide Semi conductor Field EffectTransistor (MOSFET)

Accordingly, the present invention provides a device (FIG. 5)patentdescriptionfor controlling the on & off time of the Metal OxideSemi conductor Field Effect Transistor (MOSFET) which comprises a pulsegenerator device (26) to generate packets of pulses, one terminal ofwhich being connected to the Metal Oxide Semi conductor Field EffectTransistor (MOSFET) (27), and the other terminal to a rotary selectorswitch (32), the rotary switch being used for selection of predeterminedpulses, their amplitudes and pauses referred to as “mode”, a pulsegenerator (26) also controls the selection of integrated circuit switch(I.C.switch) (33) for selection of mode through another integratedcircuit switch driver device (41), the I.C. switch (33) acting as amultiplexer and being connected to a set of variable resistors (39)through a pulse tuning device (38) and a timing capacitor (40) to apulse generator device (26) for controlling on & off time of the pulsesthat drives the MOSFET (27), the pulse tuning device (38) consisting ofa set of variable resistors (39) for each mode, positive and negativesupply being fed to variable resistors (39) for charging and dischargingpulses from pulse generator device (26).

According to another feature of the present invention there is provideda device for depositing an electrically conductive electrode material inthe form of a rod on to the surface of an electrically conductive workpiece by electrical discharges which comprises a capacitance bank (3)for charging and discharging current pulses of predetermined durationsand amplitudes, the capacitance bank (3) being connected to anelectrically conductive electrode (29) as anode, the anode being held bya vibro excitor (31) on a work piece (30) [cathode], the surface ofwhich is desired to be coated, (34) being a low inductance anode cableand (35) being a cathode cable, switching means (27) being provided witha Metal Oxide Semi conductor Field Effect Transistor (MOSFET), aprotection circuit (37) being connected to a protect MOSFET (27) againstsurge currents, one terminal of the switching means (32) being connectedto MOSFET and another to a tuning circuit (26) to control the rate ofdischarge of the current pulses of predetermined durations andamplitudes, an alternating current transformer (23) having primarywinding and three secondary windings, the primary winding being capableof being connected to an AC, 50 Hz source, all of the three secondarywindings being connected to a rectifier (24), the second one beingconnected to the vibro excitor (31) through a device (25) for generating& controlling the frequency & amplitude of the vibro excitor, the thirdwinding being connected to a device (26) for generating & controlling ofpulses to on & off time of the MOSFET (27) and the other terminal to arotary selector switch (32), the rotary switch being used for selectionof predetermined pulses, their amplitudes and pauses referred to as“mode”, a pulse generator (26) also controls the selection of integratedcircuit switch (I.C.switch) (33) for selection of mode through anotherintegrated circuit switch driver device (41), the I.C. switch (33)acting as a multiplexer and being connected to a set of variableresistors (39 )through a pulse tuning device (38) and a timing capacitor(40) to a pulse generator device (26) for controlling on & off time ofthe pulses that drives the MOSFET (27), the pulse tuning device (38)consisting of a set of variable resistors (39) for each mode, positiveand negative supply being fed to variable resistors (39) for chargingand discharging pulses from pulse generator device (26).

The mains supply voltage of 230 V is stepped down to a voltage less thanor equal to 40 V with the help of a transformer (23) having multiplesecondaries that feed a rectifier (24), an electrode holding device orvibro-excitor (25) and a pulse tuning circuit (26). The bridge rectifier(24) feeds a bank of capacitors (3), whose capacitance is in the rangeof 5000 to 150000 microfarads. The energy stored in the bank ofcapacitors (3) is discharged at the contact of electrode (29) andsubstrate (30) through MOSFET (27). Pulse tuning circuit (26) is used togenerate 1 to 3 pulses more specifically 3 as a packet by controllingvariable resistor (39) and selector switch (32) which in turn determinesthe on to off time of the MOSFET. Choke (28) is meant for producingcurrent pulses at the contact point ‘C’ of the electrode (29) and workpiece (30).

For producing a better quality of the coated layer having a lowerroughness on the processed work piece, mechanical smoothening of thesurfaces is used or an additional processed surface is produced with aprocessing electrode, using lower electrical discharges levels.

However, flattening (smoothening) of the surface by any known method canbe realized only by an additional operation, which cannot produce aquality surface with lesser roughness. Hence, a concurrent increase inthe thickness of the coated layer in a single process cycle is notpossible.

In a packet, the number of the additional current pulses, which followeach basic pulse, varies from 1 to 3. If the number of additional pulsesis more than 3, then the consequent additional efficiency of the surfacesmoothening is reduced sharply, since the roughness of the surface isalready close to the normal final value after the third additional pulseitself.

If the number of additional pulses is less than 1, i.e. 0, then thesmoothening effect may not be present at all.

The main consideration for fixing the smoothening pulse amplitude levelsto 30 to 70% of the basic current amplitude value is that this ratiopermits maximum melting and flattening (smoothening) of the processedi.e. coated surface after the primary deposition has been completed bythe basic current pulse.

If this value is reduced below 30%, then the smoothening action becomesquite insignificant. This is due to the fact that the process ofsmoothening requires careful control of the depth of melting so thatfull melting of only the rough uppermost surface of the deposited layeroccurs and current values below 30% are inadequate for this.

On the other hand, if the amplitude of the smoothening current pulse ismore than 70% of that of the basic current pulse, then the effect of theflattening/smoothening is very small, because there is hardly anydifference between the energies of the basic and additional currentpulses and therefore the roughness levels produced by them are alsocomparable, resulting in no improvement.

The pause i.e. ‘off’ time between two consecutive current pulses lastsfrom 2 to 5 times the period i.e. duration or ‘on’ time of the previouscurrent pulse, the typical current varying from 50 A to 1500 A forelectrospark coating. If this pause is less than 2 time periods of theprevious current pulse, then there is not enough time for any of themolten metal mass, produced by the previous pulse, to re-crystallize.Thus if the above criteria is not met, the depth of the molten pool fromthe start of the previous pulse till the end of the additional pulseremains the same so that there is no smoothening action produced.

If the pause duration is more than 5 time periods, then theirregularities on the processing surface get solidified in the same formand additional energy is required for heating & producing thesmoothening effect. The optimum pause between the additional pulses isequal to the time required by the melt, formed by the previous currentpulse, to crystallize.

By carrying out the spark coating in the manner described above, it ispossible to deposit any desired thickness of a material on metallic workpieces, in particular iron and its alloys, at any desired predeterminedcoating rate. The process described in the present invention obtainsdense and uniform coatings. Porosity and surface roughness in thecoatings thus obtained is reduced to significantly low levels. Thussubsequent operations such as machining or grinding are not required toremove the external rough layer, thereby resulting in cost saving.Moreover, the thickness of the coating obtained by this method issignificantly more than that obtainable by other earlier techniques.Components prepared by this process can be directly used in applicationswhere high wear and corrosion resistance is required. Further thecoatings produced by this method are more hard, adherent, smooth, denseand uniform than the coatings produced by previous methods.

With the invention described above, it is possible to deposit largeamounts of material from consumable electrode materials (coatingmaterial) on to the surfaces of work pieces that are to be coated.Moreover, the resulting surface roughness will be lower than that fromprevious methods. Higher deposition rate/pass is preferred as subsequentpasses though increase the thickness of the deposited coating theincrease in roughness is very high. This was due to the fact that theanode to cathode gap is so low that the surface irregularities alonewill grow at the expense of the surrounding area. With the method asexplained above consumable electrodes of circular cross section 0.5 to10 mm diameter and a square cross section of 0.5×0.5 mm to 10×10 mm canbe used for refurbishment of the worn components such as shafts, valvesand turbine blades.

The device of the present invention operates at very low voltages morespecifically less than 40 volts, which is in contrast to all thepreviously known circuits where researchers have employed high voltageranging 60 to 120 V or even more.

According to another embodiment of the present invention, there isprovided an improved method for depositing any electrically conductiveelectrode material in the form of a rod on to the surface of anyelectrically conductive work piece to be coated by electrical dischargesusing the above-defined device.

By employing the device of the present invention, the sparks aregenerated which raise the instantaneous temperature to a level of theorder of 10000° C., when the electric potential between the anode andcathode exceeds a critical value. A Consumable electrode in the form ofa rod is connected as the anode while the work piece who's surface to becoated serves as the cathode. As a result, the tip of the consumableelectrode melts and deposits on the surface of work piece. Melting ofthe electrode and deposition on work piece cycle is sharply controlledby the electrode repetition contact synchronized with capacitordischarge and recharge. This leads to the consumption of electrode atthe rate of 30 to 50 mg/min and while 80% of consumed electrode isdeposited on to the work piece, 20% is evaporated.

The essence of the invention lies in the fact that the main masstransfer takes place when the basic current pulse passes from theprocess electrode onto the work piece. During this pulse period, a layerof corresponding thickness is deposited on the work piece. The action ofthe additional pulses is to produce electrospark smoothening of theroughness in the layer, which is formed by the main pulse. Additional‘smoothening’ pulses with decreasing amplitudes therefore follow themain operating current pulse, their main function being to smoothen outthe roughness.

The production of high current pulses may be accomplished at the rate of100 to 12000 packets per second. Negative pole is connected to the workpiece i.e. cathode who's surface is to be coated and positive terminalis connected to the electrode i.e. anode which is to be depositedthrough a vibro-excitor which oscillates at a frequency of 50 to 300 Hzand 3 to 20 volts amplitude.

The following Examples illustrate the ability of the process using thedevice of the present invention, which are given only for the purposesof illustration and therefore should not be construed to limit the scopeof the invention.

EXAMPLE 1

An electrode 4×4×30 mm size produced from tungsten carbide with 8%cobalt (WC-8Co) is employed for deposition of coating on the surface oflow carbon steel sample with 6 mm dia.×30 mm length dimensions. Thesample is intended for the measurements of deposition rate,microhardness, surface roughness and performance evaluation of thecoating-using pin on disk method as per ASTM G65 specification. Coatingis applied in ambient air at pulse amplitudes of 850, 560, 560 amps. andeach pause equals to twice the duration of the previous pulse for1^(st), 2^(nd), and 3^(rd) pulses of the packet of pulses respectively,35 V potential, 100 Hz vibrator frequency and at a deposition rate of 1cm²/min. The average coating deposition rate, surface roughness andmicrohardness measured from the sample are 25 mg/pass/min (approx. 130μm thickness), 10-12 μm R_(a) and 1250 Hv_(0.02) respectively. Thedeposition rate mentioned above is more than 10 times faster than theknown prior art. The coating resulting from the above method was foundto exhibit a fully dense layer with very good adhesion to the substrateand 90% continuity.

Subsequently the same sample is used for performance evaluation inrelative wear assessment of coated sample vis-á-vis uncoated low carbonsteel sample of the same dimensions at a sliding velocity of 5.75 m/secand 30 N normal load against a rotating hard sintered WC-6Co disk. Theresulting steady state sliding wear loss for the coated sample ismeasured to be 1100 times lower than the uncoated low carbon steelsample.

The above results clearly illustrate the fact that the carbide coatingsobtained by the method using the device of the present invention resultsin excellent improvement in wear resistance of the components producedfrom low carbon steel.

EXAMPLE 2

Colmonoy 6 electrode, 4 mm diameter×50 mm length (Cr 14.3, B 3.0, Si4.25, Fe 4.0, C 0.70 and Ni rest)) is employed for deposition of coatingon the surface of low carbon steel sample with 6 mm dia.×30 mm lengthdimensions. The sample is intended for the measurements of depositionrate, microhardness, surface roughness and performance evaluation of thecoating-using pin on disk method as per ASTM G65 specification. Coatingis applied in inert atmosphere at a flow rate of 20 cfm, current pulseamplitudes of 650, 450, 450 amps. and each pause equals to thrice theduration of the previous pulse for 1^(st), 2^(nd), and 3^(rd) pulses ofthe packet of pulses respectively, 35 V potential, 100 Hz vibratorfrequency and at a deposition rate of 1 cm²/min. The average coatingdeposition rate, surface roughness and microhardness measured from thesample are 47 mg/pass/min (approx. 300 μm thickness), 4-8 μm R_(a) and1060 Hv_(0.02) respectively. The coating resulting from the above methodis found to exhibit a fully dense layer with very good adhesion to thesubstrate and 90% continuity. Subsequently the same sample is used forperformance evaluation in relative wear assessment of coated samplevis-á-vis uncoated low carbon steel sample of the same dimensions at asliding velocity of 5.75 n/sec and 30 N normal load against a rotatinghard sintered WC-6Co disk. The resulting steady state sliding wear lossfor the coated sample is measured to be 4 times lower than the uncoatedlow carbon steel.

EXAMPLE 3

Wallex 1 electrode, 4 mm diameter×50 mm length (Cr 33, W 12.5, Si 1.25,Fe 3.0, Ni 3.0, C 2.30 and Co rest)) is employed for deposition ofcoating on the surface of low carbon steel sample with 6 mm dia.×30 mmlength dimensions. The sample was intended for the measurements ofdeposition rate, microhardness, and surface roughness and forperformance evaluation of the coating-using pin on disk method as perASTM G65 specification. Coating is applied in inert atmosphere at a flowrate of 20 cfm, current pulse amplitudes of 650, 650, 650 amps. and eachpause equals to thrice the duration of the previous pulse for 1^(st),2^(nd), and 3^(rd) pulses of the packet of pulses respectively, 35 Vpotential, 100 Hz vibrator frequency and at a deposition rate of 1cm²/min. The average coating deposition rate, surface roughness andmicrohardness measured from the sample are 32 mg/pass/min (approx. 200μm thickness), 6-10 μm R_(a) and 910 Hv_(0.02) respectively. The coatingresulting from the above method is found to exhibit a fully dense layerwith very good adhesion to the substrate and 90% continuity.Subsequently the same sample is used for performance evaluation inrelative wear assessment of coated sample vis-á-vis uncoated low carbonsteel sample of the same dimensions at a sliding velocity of 5.75 m/secand 30 N normal load against a rotating hard sintered WC-6Co disk. Theresulting steady state sliding wear loss for the coated sample ismeasured to be 2 times lower than the uncoated low carbon steel sample.

Thus spark coating device and the method of coating using the saiddevice of the present invention is capable of depositing metalliccoatings having thickness range of 10 to 300 micrometers. Furthermore,these coatings are applied by a very low heat input process and hencefound not affecting the microstructure of the substrate/work piece.Consequently, these coatings can be used for rebuilding, repairing andrestoring the worn components produced from electrically conducting workpiece and low carbon steel in particular.

It would be obvious for anyone having a reasonable working knowledge inthis field those additional modifications and changes can beincorporated in the invention disclosed. Accordingly, such modificationsand changes are also covered within the scope of the present invention.

ADVANTAGES OF THE INVENTION

1. The working voltage for the method of coating any electricallyconductive materials on the surface of any electrically conducting workpiece using the device of the present invention is very low and henceoperator safety is fully ensured

2. Since a multiple pulse train known as “packets” is employed fordischarging the stored capacitor energy in carrying out the method ofcoating using the device of the present invention, more uniform andcontinuous coating with lower surface roughness is achieved

3. Because of the usage of multiple pulse train or “packets”, in themethod of coating using the device of the present invention lowercurrent amplitudes are able to produce deposition rates which arecomparable or better than that of the prior art known methods

4. The use of lower current amplitudes in the method of coating usingthe device of the present invention results in lower heat inputs at thesubstrate and electrode holders. Therefore it results in no ornegligible heat affected zone (HAZ) at the interface of coating and thesubstrate/work piece.

5. Coatings obtained by the method using the device of the presentinvention are uniformly dense and bonded well with the substrate.

6. Coatings obtained by the method using the device of present inventionare relatively smoother than those obtained by the prior art knownmethods.

7. Engineering components coated by the method using the device of thepresent invention can be directly used for wear and corrosion resistantapplications. Therefore costlier post coating operations such asgrinding, polishing etc can be avoided.

8. Coatings produced by the method using the device of the presentinvention are harder, adherent, smooth, dense and uniform than thoseproduced by the hitherto known prior art processes.

1-10. (canceled)
 11. A device for controlling the on and off time of theMetal Oxide Semiconductor Field Effect Transistor (MOSFET) comprising apulse generator for generating packets of pulses, the pulse generatorhaving one terminal connected to the MOSFET, and another terminalconnected to a rotary selector switch, the rotary switch operative forselection of predetermined pulses, their amplitudes and pauses referredto as “mode”, the pulse generator also controlling the selection of anintegrated circuit switch (I.C.switch) for selection of mode throughanother integrated circuit switch driver device, the I.C. switch actingas a multiplexer and being connected to a set of variable resistorsthrough a pulse tuning device and a timing capacitor to a pulsegenerator device for controlling on and off time of the pulses thatdrives the MOSFET, the pulse tuning device including a set of variableresistors for each mode, a positive and a negative supply being fed tothe variable resistors for charging and discharging pulses from a pulsegenerator device.
 12. A device for depositing an electrically conductiveelectrode material in the form of a rod onto the surface of anelectrically conductive work piece by electrical discharges whichcomprises a capacitance bank for charging and discharging current pulsesof predetermined durations and amplitudes, the capacitance bank beingconnected to an electrically conductive electrode acting as an anode,the anode being held by a vibro excitor on a work piece acting as acathode, the surface of which is desired to be coated, a Metal OxideSemiconductor Field Effect Transistor (MOSFET), a protection circuitbeing connected to protect the MOSFET against surge currents, oneterminal of the switching means being connected to the MOSFET andanother to a tuning circuit to control the rate of discharge of thecurrent pulses of predetermined durations and amplitudes, an alternatingcurrent transformer having primary winding and three secondary windings,the primary winding being capable of being connected to an AC source,each secondary winding being connected to a rectifier, one secondarywinding being connected to the vibro excitor through a device forgenerating and controlling the frequency and amplitude of the vibroexcitor, another secondary winding being connected to a device forgenerating and controlling of pulses to on and off time of the MOSFETand the other terminal to a rotary selector switch, the rotary switchbeing used for selection of predetermined pulses, their amplitudes andpauses referred to as “mode”, the pulse generating device alsocontrolling the selection of an integrated circuit switch (I.C.switch)for selection of mode through another integrated circuit switch driverdevice, the I.C. switch acting as a multiplexer and being connected to aset of variable resistors through a pulse tuning device and a timingcapacitor to the pulse generating device for controlling on and off timeof the pulses that drive the MOSFET, the pulse tuning device including aset of variable resistors for each mode, a positive and a negativesupply being fed to variable resistors for charging and dischargingpulses from a pulse generator device.
 13. The device as claimed in claim12, wherein the vibro excitor oscillates at a frequency in the range of50 to 300 Hz and has amplitude in the range of 3 to 20 V.
 14. The deviceas claimed in claim 12, further including means fixed to the vibroexcitor for supplying compressed air to cool the vibro excitor
 15. Thedevice as claimed in claim 12, wherein a total capacitance of thecapacitor bank is between 5000 and 150000 microfarads; the voltagevalues of a control circuit of the vibro excitor being set between 10 to30 V; and the voltage values output by the tuning circuit being setbetween 15 to 40 V.
 16. The device as claimed in claim 12, wherein theleads of the anode and the cathode have low inherent resistance withinductance less than 1.5 micro Henry and the capacitor bank used has aninductance less than 1.5 micro Henry.
 17. A method for depositing anelectrically conductive electrode material in the form of a rod onto thesurface of an electrically conductive work piece by electricaldischarges using the device claimed in claim
 12. 18. A device forcontrolling the on and off time of a Metal Oxide Semiconductor FieldEffect Transistor (MOSFET) comprising: a pulse generator for generatingpackets of pulses that control an on and off time of MOSFET; a set ofvariable resistors for controlling the selection of the packets ofpulses, their amplitudes and pauses generated by the pulse generator;and means responsive to the pulse generator for selecting a subset ofthe resistors thereby selecting the packets of pulses, their amplitudesand pauses generated by the pulse generator.
 19. A device for depositingelectrically conductive electrode material onto the surface of anyelectrically conductive work piece comprising: means for applying aplurality of current pulses between the electrode material and the workpiece, wherein each pair of adjacent current pulses is separated in timeby a pause during which no current flows between the electrode materialand the work piece; and means for controlling the duration of eachcurrent pulse and the duration of each pause between adjacent currentpulses such that the duration of each pause is between two and fivetimes the duration of the current pulse immediately preceding saidpause.
 20. The device of claim 19, wherein the means for controllingcontrols the duration of each current pulse such that: a first currentpulse of the plurality of current pulses has a first amplitude; and asecond current pulse of the plurality of current pulses has a secondamplitude, wherein either the first and second amplitudes are the sameor the second amplitude is less than the first amplitude.
 21. The deviceof claim 20, wherein the second amplitude is between 30% and 70% of thefirst amplitude.
 22. The device of claim 19, wherein the plurality ofcurrent pulses is no greater than 3 current pulses.
 23. A method ofdepositing electrically conductive electrode material onto the surfaceof an electrically conductive work piece comprising: applying aplurality of current pulses between the electrode material and the workpiece; and pausing between each pair of adjacent current pulses for aduration no less than twice the duration of the current pulseimmediately preceding said pause.
 24. The method of claim 23, wherein: afirst current pulse of the plurality of current pulses has a firstamplitude; and a second current pulse of the plurality of current pulseshas a second amplitude, wherein either the first and second amplitudesare the same or the second amplitude is less than the first amplitude.25. The method of claim 24, wherein a third current pulse of theplurality of current pulses has a third amplitude that is the same asthe second amplitude.
 26. The method of claim 24, wherein the secondamplitude is between 30% and 70% of the first amplitude.
 27. The methodof claim 23, wherein the plurality of current pulses is no greater than3 current pulses.
 28. The method of claim 23, wherein the pause betweeneach pair of adjacent current pulses is no greater than five times theduration of the current pulse immediately preceding said pause.